Mine got very vocal before they began laying and then they'd be on their nest making the Bawking egg song. If you are seeing them spending time in their nest and you've got a squatter then it could be any minute!
I have one GC that began laying last week (again, one week from first squat) and she has a pale smaller comb and wattles. She's laid faithfully every day since...even laying one JUMBO egg in there. Now to get the other two laying. One is squating so should be soon.
